Holdo being introduced as a leading heroic character then killed later…

Luke being killed for no reason…

Resistance contact on Canto Bight set up as an aide to the protagonists, but ending up being a useless rich brat…

That Del Toro character being introduced as an aide, then is apparently killed (?) after revealing himself to be a full-evil traitor…

Snoke being set up as an immensely powerful bad guy, but later killed without any more explanation of who/what he was…

…actually it’s hard to find a plot thread that was NOT severed in the same movie. Besides Broom Kid who surprisingly wasn’t killed, lol